With normally aspirated engines, fuel vapors are purged from a canister by utilizing the intake manifold's vacuum pressure to draw air through the canister. With turbocharged engines, there is often a positive manifold pressure generated during boost and thus there is no vacuum to draw air through the canister. Therefore, it is necessary to provide means to produce an air moving pressure differential with atmosphere so that air can be drawn from the canister to the intake manifold and be directed to the combustion chamber, thereby purging the fuel vapors by burning.
A venturi tube or nozzle is used to generate a vacuum on a turbocharged vehicle engine by scavenging from the pressure differential across the turbo (14 psi or more) to drive air through the a venturi nozzle from the turbocharger outlet and back into the turbocharger inlet. The high velocity airflow and sonic shock waves in the venturi nozzle generate a pressure lower than atmospheric (vacuum) which is used to draw purge air flow into the scavenged turbo loop.
Typically this conventional venturi and scavenging loop is an isolated loop of plumbing, requiring a tube from the vacuum port on the venturi nozzle to a port on the purge valve. The purge valve is protected from purge loss during naturally aspirated conditions and from turbo pressures by a check valve located between the venturi nozzle and the purge valve, and another check valve located between the intake manifold and the purge valve respectively. Thus, such an arrangement requires multiple plumbing connections and discrete components that increase cost.
There is a need to provide a compact canister purge valve for a turbocharged engine, with canister purge valve having an integrated vacuum generator and check valves.